1. Field of the Invention.
The present invention relates to a method for producing hematite for ironmaking. Particularly, the present invention relates to a method for producing hematite for ironmaking which can efficiently suppress the mixing of a sulfur compound into a leach residue in real operation by providing a plural series of solid-liquid separation steps in the hydrometallurgical process of nickel oxide ore.
2. Description of the Related Art.
Nickel has been widely used as a raw material of stainless steel.
However, with a tendency of depletion of a sulfide ore as a raw material of nickel, a technique of refining low-grade oxide ore has been developed and put into practical use.
Specifically, a production process called “high pressure acid leach (HPAL)” has been put into practical use, in which nickel oxide ore such as limonite and saprolite is put in a pressure device such as an autoclave together with a sulfuric acid solution, and nickel is leached under a high temperature of about 240 to 260° C. and high pressure.
To a solution containing nickel leached into the sulfuric acid solution, is added a neutralizing agent to neutralize excess acid, and then the resulting mixture is subjected to solid-liquid separation to separate nickel from a leach residue. Subsequently, impurities are separated to recover nickel as an intermediate material in the form of hydroxide, sulfide, or the like. The intermediate material is further refined to thereby obtain nickel metal, a nickel salt compound, or the like.
Note that in the preliminary neutralization step of neutralizing the excess acid, the solution is adjusted to a pH suitable for solid-liquid separation, and in the next step of solid-liquid separation step, the concentration of solids and solid-liquid separation are performed in a facility called CCD (Counter Current Decantation). Generally, a plurality of successive stages of thickeners are used in CCD.
A liquid component (hereinafter may be referred to as an overflow) obtained from the CCD is transferred to a neutralization step so as to be adjusted to a pH suitable for a sulfurization step, and fine solids produced by adjusting pH are removed by precipitation. Then, the resulting liquid component is generally sent, for example, to the sulfurization step to be subjected to sulfurization treatment to produce an intermediate material, that is, a mixed sulfide of nickel and cobalt.
In this case, Japanese Patent Laid-Open No. 2004-225120, for example, describes a technique of adding a part of solids obtained by CCD (hereinafter may be referred to as an underflow) to the neutralization step as seed crystals to accelerate the production of fine precipitates, and the technique is effectively utilized to improve the efficiency of real operation.
In such a production process called high pressure acid leach (HPAL), nickel can be leached substantially completely, for example in the case of nickel oxide ore, even if the ore is a low grade ore containing a target valuable metal to be recovered in an amount of 1 to 2% by weight or less (hereinafter, the grade is represented by “%”).
Further, by producing an intermediate material from a leachate, a target metal can be obtained by concentrating the target metal to the same degree as in a conventional raw material and employing substantially the same refining method and steps as for a conventional raw material.
Further, the HPAL process can be applied not only to nickel oxide ore but to various ores such as nickel sulfide ore, copper sulfide ore, and copper oxide ore.
Further, the main component of the leach residue obtained by the HPAL process is iron oxide in the form of hematite and the like. The content of iron in the leach residue is about 50%, and the production volume of the leach residue is about 50 times to 100 times the production volume of the intermediate material. This is because the nickel oxide ore and the copper sulfide ore used as raw materials contain iron in an amount far exceeding the content of nickel and copper, respectively.
The leach residue is in the form of a chemically and environmentally stable oxide since it is produced at high temperatures, but under the present circumstances, it does not have particular utility value and is accumulated and stored in a residue stacking yard.
Therefore, a wide residue stacking yard is required for accumulating and storing a huge amount of leach residue produced with the operation of the HPAL process.
On the other hand, in steel smelting, there is used a method including charging iron ore containing iron oxide into a blast furnace together with a reducing agent such as coke, heating the iron ore to reduce and melt it to obtain crude steel, and refining the crude steel in a converter to produce target steel.
Iron oxide as the raw material is limited resources, and, moreover, it is increasingly difficult to acquire good quality iron ore required for maintaining the quality of steel. Therefore, investigation of using the leach residue as iron ore has been made.
However, it has been unable to directly use the leach residue from the HPAL process for a raw material for ironmaking.
The reason includes the following two points.
(1) Since the leach residue from the HPAL process contains vein stone and impurities, particularly sulfur in addition to iron oxide, it has not been suitable as a raw material used for conventional common ironmaking processes.
(2) The average particle size of hematite recovered from the leach residue is as very fine as 1 μm or less, and its handling is difficult.
Incidentally, the iron ore can be classified into lump ore (6.3 to 31.5 mm), powder ore (1 to 6.3 mm), and fine powder ore (0.05 to 0.1 mm), and the raw material for ironmaking which is charged into a blast furnace includes three types, that is, lump ore, sintered ore, and pellet.
Note that sintered ore is produced by sintering powder ore, and pellet is obtained by baking fine powder ore.
The sulfur impurity in the iron oxide that can be utilized as a raw material for ironmaking, which is different depending on the equipment capacity of each iron mill, production volume, and the like, is generally required to be suppressed to less than 1%.
On the other hand, the leach residue from the HPAL process normally contains about 5 to 8% of sulfur.
Most of the sulfur in the leach residue is derived from calcium sulfate (gypsum) mixed during nickel refining.
When free sulfuric acid (free sulfuric acid means sulfuric acid remaining unreacted in the sulfuric acid excessively added to perform sufficient leaching in the HPAL process) remaining in the leach slurry obtained in high pressure acid leach is neutralized, a neutralizing agent that is common, inexpensive, and calcium-based, for example, limestone or slaked lime, is added. The gypsum is produced by the reaction between the calcium contained in the neutralizing agent and the free sulfuric acid and mixed into the leach residue.
Note that a part of sulfur (about 1%) contained in the leach residue is incorporated into hematite particles produced.
Therefore, it is considered preferable to use a neutralizing agent as an additive which is not an agent forming a poorly soluble precipitate after neutralization, such as limestone or slaked lime, but an agent producing a soluble salt.
Examples of neutralizing agents suitable for such application include sodium hydroxide, potassium hydroxide, magnesium hydroxide, and magnesium oxide.
However, these neutralizing agents are not suitable for the process of consuming a large amount of neutralizing agents, such as the HPAL process, because these neutralizing agents are expensive or produced in a small amount.
Therefore, since it is forced to use, totally or partially, the calcium-based neutralizing agent which forms a poorly soluble precipitate after neutralization as described above and the mixing of sulfur cannot be avoided, it has not been possible to process the leach residue produced in the HPAL process into hematite to be used as a raw material for ironmaking.
On the other hand, there is also known a method of using a pressure device such as an autoclave to separate sulfur in jarosite.
For example, Japanese Patent Laid-Open No. H03-176081 discloses a method comprising stirring a jarosite-containing residue and a zinc sulfide-containing concentrate in an autoclave at an oxygen partial pressure of at least 1000 kPa and a temperature of 130 to 170° C. together with 40 to 100 g/L of free sulfuric acid to substantially dissolve iron and zinc in the residue and the zinc sulfide-containing concentrate, introducing the resulting solution into a leach circuit for zinc electrolysis to precipitate iron in the form of hematite to separate sulfur from the solids, and supplying the residue to another application.
However, this method requires an expensive new device such as an autoclave and an increase in equipment cost, and has had a problem also in productivity.
Therefore, the use of magnesium oxide that is contained in the ore itself as a neutralizing agent has been proposed.
For example, Japanese Patent Laid-Open No. H03-176081 discloses a process of recovering magnesium oxide from a source of magnesium sulfate, the process including the steps of: providing a source of magnesium sulfate in solution that is derived from part of a process associated with the leaching of a metal containing ore or concentrate; converting the magnesium sulfate in solution to solid magnesium sulfate; contacting the solid magnesium sulfate with elemental sulfur in a reducing atmosphere; and recovering magnesium as magnesium oxide, and sulfur as sulfur dioxide gas.
By using this method, the magnesium contained in the ore can be reused as a neutralizing agent, and calcium to be incorporated can be suppressed. As a result, calcium that is mixed into the iron oxide in the residue can be reduced.
However, the method of Patent Literature 3 requires a large amount of heat for crystallizing magnesium in the solution as magnesium sulfate and heating the resulting magnesium sulfate to convert it into magnesium oxide, and the method cannot be said to be an economical method.
Further, there is also proposed a method of using an oxide ore having a high magnesium content (limonite ore) as a neutralizing agent.
For example,Japanese Patent No. 4294685 discloses a method of recovering nickel or cobalt from an oxide ore containing nickel or cobalt and iron, the method comprising: a step of providing a first oxide ore and a second oxide ore having a higher magnesium content than the first oxide; a classification step of classifying the first oxide ore to a first small particle size oxide ore and a first large particle size oxide ore and classifying the second oxide ore to a second small particle size oxide ore and a second large particle size oxide ore; a leaching step of leaching nickel or cobalt from the first large particle size oxide ore with sulfuric acid to obtain a sulfuric acid leach solution containing nickel or cobalt and a leach residue from the first large particle size oxide ore; a reaction step of mixing the sulfuric acid leach solution containing the leach residue and the second large particle size oxide ore to allow the sulfuric acid leach solution and magnesium contained in the second large particle size oxide ore to react with each other and adjusting the pH of the reaction mixture to obtain a reaction mixture containing nickel or cobalt and a reaction residue containing iron; and a neutralization step of neutralizing the reaction mixture containing the reaction residue with a neutralizing agent to obtain a neutralized solution containing nickel or cobalt and a neutralization residue containing iron.
The nickel oxide ore itself can be utilized as a neutralizing agent by using this method.
However, the cost and the time and efforts for classifying ores have not been able to be ignored. Further, a high level of gangue component is present in the leach residue, and when the leach residue is used as it is, the iron grade will be low. Thus, it has not been an efficient raw material.
Therefore, it has been difficult to replace the whole amount of the neutralizing agent used in the HPAL process with magnesium oxide.
Further, a method is easily remembered in which the neutralizing agent is replaced with magnesium oxide derived from a base rock only in a preliminary neutralization step of producing a leach residue to prevent the mixing of sulfur.
However, when a conventional calcium-based neutralizing agent is used in a neutralization step to utilize the technique of improving the efficiency of real operation described in Japanese Patent Laid-Open No. 2004-225120, the residue from the neutralization step will be returned to CCD. Therefore, the mixing of sulfur into the leach residue cannot be avoided, and a new problem of causing an increase in sulfur impurity will arise.
In such a situation, the present invention provides a method for producing hematite from a leach residue containing fine iron oxide produced in the HPAL process, firstly the method allowing the use of conventional Ca-based neutralizing agents and neutralizing agents, other than Ca-based neutralizing agents, which are derived from a base rock, in real operation of refining hematite to obtain hematite containing a low level of sulfur component to such an extent that the hematite can be used as a raw material for ironmaking, and secondly the method allowing the sintering of very fine powder ore which has not conventionally been used.